Cloth on an armature are deforming with ripped edges when armature pose is changed
The problem is the mesh of the clothes. It's solidified (2 faces, inside and outside), and quite dense. The mesh is also separated (sleeves front &amp; back, shirt front &amp; back, pants legs front, ...). This messes up the weight paint and the armature can't deform the clothes properly.
For animation, you want a rather low-poly mesh with a baked normal map for the wrinkles. Bake it from the high-poly version with the cloth simulation. Already 10-15k vertices can look nice. In Blender, you can add a Solifiy and Subdivision Surfaces modifier after the Armature modifier if more geometry is needed. This will not mess up the weight paint and animation.
If you have the source for the simulated clothes in another blend file you can add it to your scene (menu: File → Append...) and work with this.
If you have only the FBX file to import it.... well, that's bad because you have to fix it manually and this is some work.
There are several issues with the clothes:

the mesh is solidified. That's the worst problem. It causes this &quot;underlay&quot; issue. Because of this, you would need to weight-paint it inside and outside. Fix: not sure, remove the inside manually?
it's a high-poly, dense mesh and has a Subdivision Surface modifier on top of this! This boosts the number of vertices to about 700,000! Such a high poly count slows down the animation a lot and is not needed. Fix: Remove the Subdivision Surface modifier
there are 12,399 duplicated vertices. Fix: Go in Edit mode, select all vertices (A), and do a Merge By Distance (M)
the buttons of the clothes have &quot;threads&quot; that consist of 2,736 loose vertices (no faces, no edges, just a heap of verts). For some reason I don't know you cannot get rid of them by using the menu Mesh → Clean Up → Delete Loose (Blender 2.92.0). Fix: But you can select them all with the menu entry Select → Select All by Trait → Loose Geometry, and delete them (X, Vertices). If you want threads you can model them easily with a few vertices (it's just an &quot;X&quot;)


After the easy clean-up, the mesh has about 160,000 vertices. If you want to improve it more you need &quot;un-solidify&quot; it. For the shirt is can be done by adding UV seams at the bottom of the sleeves and the neck (in Edit mode, select the edge loops, then U, Mark Seam), then you can select the inner faces when you hover the mouse pointer over the mesh and press L. Finally, delete it.
But the pants are a pain because at the cloth seams the mesh forms a cylindric shape that is inside of the mesh. The worst is that it is connected to the outside (red on the screenshot) and shares some faces. I don't know how you can rid of this easily. Doing it manually takes a lot of time.
View from inside of the pants, at the vertical cloth seam with inner faces